After you complete Wayward Son you can pick this quest up at Gamlen's House from the Writing Desk. Head towards the Wounded Coast to find the Runaway's Cave. You'll find Thrask waiting outside the cave for you with a request, accept his request and go inside. You'll have to battle a bunch mages in the cave.

After killing Decimus, Grace will approach you. You have a few options when dealing with her:

You can refuse to help and the apostates will leave the caves, but they'll threaten to expose the mages in your party. You can avoid fighting Karras with this option.

You can lie to Thrask and say they're all dead, and again you might be able to avoid killing Karras.

You can help the mages, kill Thrask and the rest of the templars, including Karras.

Use Varric if you're trying to avoid a fight with the templars and killing Karras. If you do kill Karras, you'll unlock The Midnight Meeting quest in Act 2. If the mages go to the Circle, you'll get the Bounty Hunter quest from the Chanter's Board in Act 2, and if the mages go free, you'll get the Search and Rescue quest in Act 2. You'll also get a bonus item, the Chantry Staff, if you help the mages.

You'll get points with Merrill if you say "I'm sorry" when you're talking to Grace after killing Decimus. If you promise to kill Thrask, you'll get some points from Anders, neutral from Bethany, and negative points from everyone else. If you turn the mages over, Anders, Bethany and Merrill will get pissed, but Aveline, Carver and Fenris will approve. The best outcome from the companion point of view is to lie, this won't change much except a boost to Fenris.